Field of the Invention
This invention relates to an edge light panel device of a surface light source device to be used for various illuminating apparatus such as a back light of a liquid crystal display, illuminated display or illuminated signboard, etc..
A surface light source device of this type normally comprises one or more primary light sources, e.g., fluorescent lamps, arranged respectively along an edge or edges of the edge light panel so that the light from the primary light sources is supplied to the edge light panel in order to evenly illuminate it and to serve as a surface light source.
An edge light panel device of the type under consideration normally comprises a single- or multi-layered transparent substrate having a thickness of several millimeters, typically five to six millimeters, and made of a material having a high light transmittivity such as acrylic resin, whose surface, normally its back side, is treated by screen printing with white ink to carry a pattern formed by a large number of dots with different surface areas for irregular reflection of light. The dots are arranged in such a manner that they show a smooth horizontal or vertical transition within the pattern, in order for the light beams led into the transparent substrate to be irregularly reflected within the substrate. In addition, a surface diffusion layer made of a white or milky white reflective sheet is normally arranged on both the front and back sides of the edge light panel to diffuse the light beams irregularly reflected within the panel and enhance the brightness of the edge light panel, and the surface diffusion layer on the back side is lined by a white or milky white reflective sheet or a metal foil to prevent the irregularly reflected light within the surface diffusion layer from escaping.
While a surface light source device as described above ensures a certain degree of even brightness, it is apt to produce round regions near the edges adjacent to the primary light source where the brightness level is lower than in the remaining regions particularly when the thickness of the transparent acrylic resin substrate used for the edge light panel is reduced from 2 to 3 millimeters relative to known substrates to 1.5 millimeters or less. Moreover, the means for irregular reflection of light, typically screen printed dots, may become visible through the surface diffusion layer to damage the evenness of the brightness of the surface light source panel as the transparent substrate is made thin.
While these problems may be solved to some extent by preparing a relatively thick surface diffusion layer by bonding two white sheets together, such a thick surface diffusion layer can significantly reduce the brightness of the surface light source device and is not feasible for practical applications.
Since an edge light panel device or a surface light source device comprising an edge light panel is often used for a back light of a liquid crystal display and installed in an apparatus containing the device and the display along with a keyboard, it is most desirable to have a very thin surface light source device in order to make the entire apparatus very small and compact. However, known edge light panels are unable to meet the requirements because of the problems as described above.